Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and servo sectors. The servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the velocity of the actuator arm as it seeks from track to track.
Because the disk is rotated at a constant angular velocity, the data rate is typically increased toward the outer diameter tracks (where the surface of the disk is spinning faster) in order to achieve a more constant linear bit density across the radius of the disk. To simplify design considerations, the data tracks are typically banded together into a number of physical zones, wherein the data rate is constant across a zone, and increased from the inner diameter zones to the outer diameter zones. This is illustrated in FIG. 1A, which shows a prior art disk format 2 comprising a number of data tracks 4, wherein the data tracks are banded together in this example to form three physical zones from the inner diameter of the disk (ZONE 1) to the outer diameter of the disk (ZONE 3).
The prior art disk format of FIG. 1 also comprises a number of servo sectors 60-6N recorded around the circumference of each data track. Each servo sector 6i comprises a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a track address, used to position the head over a target data track during a seek operation. Each servo sector 6i further comprises groups of servo bursts 14 (e.g., A, B, C and D bursts), which comprise a number of consecutive transitions recorded at precise intervals and offsets with respect to a data track centerline. The groups of servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations.
Multiple access commands may be received from the host while the disk drive is executing a current access command. The access commands are typically buffered in a command queue, and when the disk drive is finished executing the current command, a next command is selected from the command queue according to a rotational position optimization (RPO) algorithm. The RPO algorithm attempts to select the next command that will minimize the mechanical latency in accessing the disk, including the seek time required to move the head to the target track and the rotational time for the head to reach the target data sector.
The seek time is typically estimated for a given seek length (the number of tracks the head must traverse to arrive at the target track), where the seek time is typically related to the performance capabilities of the VCM as well as the parameters selected for the servo system to perform the seek, such as the selected velocity trajectory. Since the performance capability of the VCM is typically not a linear function of seek distance, and since the servo parameters may change based on the seek distance, the relationship between the seek length and seek time is also typically not linear across the radius of the disk. This is illustrated in FIG. 1B which shows a seek profile giving the seek time as a function of seek length. In this example, the seek profile is divided into a number of seek length segments, where each seek length segment may correspond to adjusting a servo system parameter, such as adjusting the velocity trajectory. Each seek length segment may correspond to a single seek time estimate, or each seek length segment may correspond to a function (e.g., a linear function) which may provide a more accurate estimate of the seek time relative to the seek length.
Estimating the seek time based on a segmented seek profile as shown in FIG. 1B may not be particularly accurate due to the variance of the seek time across the disk radius independent of each seek length segment. To compensate for the variance, a margin is typically associated with each seek length segment to ensure that the estimated seek time does not underestimate the actual seek time which can significantly degrade performance due to slipped revolutions. However, for certain seek lengths the margin is too conservative which can also degrade performance by preventing the most optimal command from being selected.
There is, therefore, a need to improve the seek time estimates for an RPO algorithm employed in a disk drive.